Method and apparatus for ultrasonic testing of the surface of columnar structures, and method for grinding rolls by use of them

ABSTRACT

An ultrasonic testing method uses surface waves to prevent false detection of primary cracks, to lower the level of structural noises from grain boundaries, and to improve the detectability in the ultrasonic testing by surface wave testing of hot rolling rolls. A surface wave probe  10  capable of transmitting and receiving a surface wave is provided with a piezoelectric element  10 A, a resin wedge  10 C disposed on the front surface of the piezoelectric element  10 A and a damping block  10 B disposed on the back surface. The surface wave probe  10  is driven to produce a short pulse having a pulse length being at most 2.5 times the wavelength of the surface wave to be produced. A coupling liquid medium is supplied to the probe  10  in accordance with the peripheral speed of the roll to be tested. Depending on the height of the reflected waves measured, the grinding allowance of the roll is determined, and the roll is ground according to the thus-determined grinding allowance. The roll may be tested while being partly ground, and the optimum grinding allowance of the roll may be determined.

TECHNICAL FIELD

The present invention relates to a method and an apparatus forultrasonic testing of the surface of columnar structures of metal, suchas rolls for rolling mills, rollers and others, especially to thosesuitable for detecting, using surface waves, flaws such as cracks or thelike existing in and just below the surface of high-speed tool steelrolls for hot rolling mills, which are made of high-speed tool steel andof which the surface is thermally/mechanically damaged while they areused for rolling, and also to a method for grinding rolls using them.

BACKGROUND OF THE INVENTION

Rolls for hot rolling of metal sheets are thermally/mechanically damagedat their surface while they are used for rolling. The details ofthermal/mechanical damage at the surface of a work roll (hereinafterreferred to as a high-speed tool steel roll for former stands infinishing train), which is made of high-speed tool steel and is used inhot finish rolling, are described with reference to FIG. 20. Thermaldamage to the roll is caused by steel sheets being rolled at hightemperatures in former stands in finishing train, whereby deep primarycracks K, which are referred to as fire cracks, are formed in the roll100 vertically to its surface. Mechanical damage thereto is caused bythe shear stress to be put on the roll being rolled against backuprolls, whereby are formed secondary cracks L starting from theabove-mentioned fire cracks K in the direction nearly in parallel withthe surface of the roll. A plurality of those cracks gathers to givesmall pits M on the surface of the roll. If such small pits M aretransferred onto the sheets being rolled, the rolled sheets shall havesurface flaws. In order to evade this, the cracks are removed fromrolls, for example, by means of grinding by a predetermined constantgrinding allowance with a grinder, and the thus-ground rolls are againused in rolling. After having been ground, the rolls are tested bysurface wave technique (hereinafter referred to as surface wavetesting), for example, as in JP-A-4-276547.

Concretely, a surface wave probe (search unit) is kept in contact withthe surface of a rotating roll via a membrane of a coupling liquidmedium such as water or the like, whereby the surface waves from saidsurface wave probe is propagated inside the circumferential surface ofthe roll toward the direction opposite to the direction in which theroll is rotating while the liquid having been used as coupling medium isremoved from the path of the surface waves in the surface of the roll.In that manner, the flaws, if any, existing in and just below thesurface of the roll are detected. If some flaws are detected in suchsurface wave testing of rolls, the rolls shall be again ground.

An ultrasonic test apparatus is disclosed in JP-A-7-294493, to which isapplied the testing method of JP-A-4-276547. The ultrasonic testapparatus comprises a rotating means for rotating a cylindrical orcolumnar structure to be tested for surface flaws and others, in itscircumferential direction; an ultrasonic probe for detecting flaws andothers by use of surface waves; a holder for holding the probe above thestructure to be tested at a predetermined height relative to the surfaceof the structure; and a couplant supply for supplying a liquid mediumsuch as water or the like to be a coupling medium for ultrasoundtransmission, to the gap between the probe and the structure to betested. The above-mentioned holder extends downward below the probe, andhas a following part to be in smooth contact with the surface of thestructure to be tested. While kept in contact with the rotatingstructure to be tested, the following part ensures the constant distancebetween the probe and the structure. The above-mentioned couplant supplyis disposed adjacent to the probe inside the holder. The couplant supplyis provided with a housing in which the liquid medium having been lednear the probe from the other place is stored. The housing is positionedadjacent to the probe, and has a lot of medium outlets at its bottom andan air-discharging through-hole at its top. Each of the medium outletsis placed just in front of the probe relative to the scanning directionof probe, while the surface of the structure being rotated to be testedis scanned by the probe, and is so disposed that it intersects theabove-mentioned circumferential direction. In that constitution, theliquid medium stored in the housing is fed through those medium outletsto the gap between the probe and the surface of the structure beingtested, at the place just in front of the leading edge of the probe.

Recently, however, it has been clarified that, when the testing methodaccording to JP-A-4-276547 is applied to the detection of flaws existingin and just below the surface of high-speed tool steel rolls, especiallythose for former stands in finishing train, there occur serious problemssuch as those mentioned below.

Specifically, the small pits M shown in FIG. 20 are not formed in theabsence of the secondary cracks L. Therefore, in order to prevent theformation of the small pits M, only the secondary cracks L are to beremoved through grinding. However, according to the conventionalultrasonic testing method, often observed is the phenomenon oflarge-amplitude reflected waves appearing even after all those secondarycracks L are removed. Grinding rolls until no such reflected waveappears results in the thorough removal of the primary cracks K needlessto be removed, whereby the roll consumption shall increase.

This is because, in the surface wave testing, the reflectivity of theflaws vertical to the surface of the roll being tested is high. In this,therefore, the primary cracks K remain are detected falsely even afterall the secondary cracks L are removed. The depth of the primary cracksK remain is considerably shallow, the amplitude of the wave reflected oneach primary crack K remains is very small. However, in the surface ofthe roll, there are innumerable primary cracks K remain, and there arealso innumerable reflectors by which the surface waves are reflected.Where a structure (herein roll) having such innumerable small reflectorsis tested by surface waves having a wavelength of λ, there always existcombinations of small reflectors between which the difference in thedistance from the surface wave probe 10 is λ/2, as shown in FIG. 21.FIG. 21 shows examples of the combinations of such small reflectors, inwhich small reflectors K1 to K4 correspond to the combinations.

Where the region in which the small reflectors K1 to K4 exist is testedin a conventional manner using a narrow bandwidth pulse of which thelength is at least 5 times larger than the wavelength of the resultingsurface wave, small reflected waves from those reflectors overlap inphase with each other, owing to the large pulse length, therebyenlarging their amplitude to give a large reflected wave that mayindicate the presence of just like a large flaw, as shown in FIG. 22.Specifically, since such a narrow bandwidth pulse of which the length isat least 5 times larger than the wavelength of the resulting surfacewave is used in surface wave testing of high-speed tool steel rolls forformer stands in finishing train, the amplitude of the reflected wavesfrom the primary cracks K is detected too high in the test. As a result,the rolls are to be ground until the amplitude of the reflected wavesfrom the primary cracks, K becomes lower than a predetermined voltage.Consequently, since the primary cracks K are almost completely removed,or that is, since the rolls are too much ground, the roll consumption isto increase.

In this connection, it may be taken into consideration to elevate thethreshold voltage that is settled for detecting flaws to a degree notbringing about false detection of the primary cracks K. However,elevating the threshold voltage lowers the detectabilities of the testdevice, and therefore, there is a danger of overlooking cracks and thelike which exist alone (these are produced by rolling accidents) andwhich must be detected.

The problems with the ultrasonic testing mentioned above are not relatedto only the primary cracks of rolls for rolling mills but also others.Where ordinary columnar structures of metal such as rollers and the likehaving a coarse grained structure and producing scattered waves at theirgrain boundaries are tested, using the conventional narrow bandwidthpulse, formed are high structural noises owing to the same mechanisms asthose of the phenomenon previously mentioned hereinabove. Therefore,there are the same problems as above with the ultrasonic testing of thattype.

On the other hand, the layer thickness of the thermal/mechanical damagedzone generated inside the rolls having been used in rolling greatlyvaries, depending on the length of the metal sheet rolled by the use ofthe rolls, the rolling speed, the condition for cooling the rolls, thematerial of the rolls (even rolls of the same type often differ in thedetails of the material owing to the difference in the manufacturemethod for the material), etc. Therefore, where the rolls are ground bya predetermined constant grinding allowance, there occur the problemsmentioned below.

{circle around (1)} Where the thermal/mechanical load imparted to therolls is small and the layer thickness of the thermal/mechanical damagedzone generated inside them is thin, the rolls are ground to remove eventhe part not damaged (overgrinding), or that is, the rolls are too muchground, whereby the roll consumption increases.

{circle around (2)} Where the thermal/mechanical load imparted to therolls is large and the layer thickness of the thermal/mechanical damagedzone generated inside them is thick, the damaged zone still remains evenafter the rolls are ground by a predetermined constant grindingallowance. When the thus-ground rolls are tested, some false indicationsoccur. In that case, the rolls will have to be further ground. However,since the layer thickness of the remaining thermal/mechanical damagedzone is not clarified, the grinding allowance of the additional grindingmust be the same as that of the initial grinding. In most cases,therefore, the rolls are too much ground (overground), or that is, thedegree of the total grinding is too large, whereby the roll consumptionincreases.

{circle around (3)} The overall time taken for grinding the rolls isprolonged by the time taken for the over grinding, and the roll grindingefficiency is lowered.

In the ultrasonic test apparatus disclosed in JP-A-7-294493, nothing istaken into consideration relative to the relationship between therotating speed of the structure to be tested and the necessary amount ofthe coupling liquid medium. However, depending on the rotating speed ofthe structure to be tested, the amount of the coupling liquid mediumthat is trained by the rotating surface of the structure being testedvaries. For example, with the increase in the rotating speed of thestructure being tested, the amount of the coupling liquid medium that istaken out of the gap between the probe and the surface of the structureincreases, thereby resulting in that the amount of the coupling liquidmedium to be in the gap between the probe and the surface of thestructure is short. As a result, ultrasonic waves could not be welltransmitted into the structure to be tested, and some surface flaws ofthe structure could not be detected. On the contrary, when the rotatingspeed of the roll is slow, the surplus coupling liquid medium flows outahead of the probe, thereby attenuating surface waves. In that case,some surface flaws of the structure could not be detected.

No concrete disclosure is given in JP-A-4-276547 and JP-A-7-294493,relating to the surface wave probe used therein, and therefore thedetails of the probe are unknown. However, it may be considered that aconventional known surface wave probe would be used therein. Oneconventional surface wave probe comprises a piezoelectric element, and awedge via which an ultrasonic wave is introduced into the roll surfaceat an angle of θi, in which the incident angle θi is defined to satisfythe following formula (1) according to the laws of refraction.

θi=sin⁻¹(CW/CRs)  (1)

wherein CW indicates the velocity of the ultrasonic wave in the wedge,and

CRs indicates the velocity of the surface wave traveling in ordinarysteel.

The incident angle θi is an angle to the plane vertical to the rollsurface.

Recently, however, it has been found that the flaw echoes could not havea satisfactory signal-to-noise ratio in the surface testing using theabove-mentioned surface wave probe.

Accordingly, having noted the difference in the material of high-speedtool rolls owing to the difference in the production method for thematerial, we, the present inventors have investigated how or in whatmanner the difference in the material may influence the surface wavetesting of the rolls. As a result, we have found that the surface wavevelocity on rolls of different materials greatly differs depending onthe production method, as in Table 1.

The inventors have further found that the surface wave velocity onhigh-speed tool steel rolls significantly differs from that on ordinarysteel of being 2980 m/sec.

Transmission and receipt of surface waves in the surface of a roll by asurface wave probe is effected according to the refraction phenomenonexpressed in the above-mentioned formula (1), while the incident angleθi is so defined that the angle of refraction is to be 90 degrees.Therefore, when the velocity of the surface waves varies depending onthe roll materials, as in Table 1, the incident angle must be varied inaccordance with the varying velocity of the surface waves. If not,transmission/receipt efficiency of the surface waves will lower.

At present, however, the incident angle θi is determined on the basis ofthe surface wave velocity on ordinary steel of being 2980 m/sec, andsurface wave probes are designed in accordance with the thus-determinedincident angle. In fact, as shown in Table 1, the surface wave velocityon different high-speed tool steel rolls significantly differs from thaton ordinary steel. In addition, there are differences in the surfacewave velocity between different high-speed tool rolls owing to thedifference in the production method for the rolls, but, in fact, thedifferences in the surface wave velocity are not taken intoconsideration at all in designing the incident angle θi. As a result,the incident angle θi greatly differs from the optimum angle fortransmission and receipt of the surface waves.

Ultrasonic testing of high-speed tool steel rolls is generally carriedout at or around a roll grinding equipment. In many cases, themechanical equipment often generates great electric noises from motors,inventors, etc., and the great electric noises are often superimposed onsignals of surface wave testing. In that case, sincetransmission/receipt efficiency of the surface waves is really loweredin the surface wave testing for high-speed tool steel rolls, as somentioned hereinabove, the height of the reflected waves from the flawsis lowered, thereby resulting in that the signal-to-noise ratio,echo-amplitude/electric-noises-amplitude is lowered. As a result, thedetectabilities of the surface wave testing are lowered.

Even when the incident angle θi is determined in accordance with thesurface wave velocity on a high-speed tool steel roll of a specificmaterial and a surface wave probe is manufactured on the basis of thethus-determined incident angle, and when the surface wave velocity isdefined to be equal to, for example, any of the smallest or largestvalue of 3090 m/sec or 3180 m/sec in Table 1, transmission/receiptefficiency of the surface waves for the high-speed tool steel rollhaving a largest or smallest surface wave velocity oppositely to thedefined value is still lowered, and, in that case, the detectability inthe testing are lowered. Thus, this could not still solve the problem.

Every time exchanging the surface wave probe to the best one for eachroll of a different material requires a prolonged time for testing,thereby probably causing time loss in actual operation, and such isimpracticable.

The present invention has been made so as to solve the problems in theconventional art noted above, and its first subject matter is to providea method and an apparatus for ultrasonic testing of columnar structuresusing surface waves, in which false detection of primary cracks isprevented and the level of structural noises from grain boundaries isreduced, and of which the detectabilities are enhanced.

The second subject matter of the invention is to provide an ultrasonictest apparatus in which a suitable amount of a coupling liquid medium isalways supplied to the gap between a probe and the surface of astructure to be tested even when the rotating speed of the structure isvaried, thereby maintaining good transmission of ultrasonic waves to thestructure and preventing any surplus coupling liquid medium from flowingout ahead of the probe.

The third subject matter of the invention is to provide a method forgrinding a roll having been thermally/mechanically damaged on itssurface in use for rolling or the like, in which the grinding allowanceof the roll is optimized to reduce the roll consumption and to improvethe roll grinding efficiency.

The fourth subject matter of the invention is to provide a method forultrasonic testing of high-speed tool steel rolls using surface waves,in which is used a surface wave probe for roll testing capable ofensuring efficient transmission and receipt of surface waves in therolls and capable of increasing the signal-to-noise ratio of thereflected waves from the flaws, even when the materials of the rollsdiffer owing to the difference in the production method, and in whichthe incident angle for the probe is specifically defined.

DISCLOSURE OF THE INVENTION

The invention is a method for ultrasonic testing of columnar structures,in which a surface wave probe is contacted with the surface of arotating columnar structure via a coupling liquid medium existingtherebetween, and a surface wave is propagated into the columnarstructure from the surface wave probe while the probe receives thereflected waves from the flaws existing in and just below the surface ofthe columnar structure so as to detect the flaws. In, the method, wherethe center frequency of the surface wave, to be transmitted and receivedby the surface wave probe is fc in the frequency spectrum, the frequencybandwidth within which the spectrum magnitude falls the range betweenthe peak value and the peak value−6 dB covers at least 0.50 fc orlarger. The method of the invention has attained the above-mentionedfirst subject matter.

In the ultrasonic testing method of the invention to attain theabove-mentioned first subject matter, the pulse length of the surfacewave pulse which the surface wave probe transmits and receives is atmost 2.5 times the wavelength. of the surface wave that propagates onthe columnar structure.

The invention is also an apparatus for ultrasonic testing of columnarstructures, in which a surface wave probe is contacted with the surfaceof a rotating columnar structure via a coupling liquid medium existingtherebetween, and surface waves are propagated into the columnarstructure from the surface wave probe while the probe receives thereflected waves from the flaws existing in and just below the surface ofthe columnar structure so as to detect the flaws. In the apparatus, thesurface wave probe that transmits and receives the surface wave isprovided with a wedge as disposed on the front surface of thepiezoelectric element of the probe and with a damping block as disposedon the back surface of the piezoelectric element. The apparatus of theinvention has attained the above-mentioned first subject matter.

In the invention, the piezoelectric element material is any of a leadmeta-niobate, a lead titanate, a 1-3 type piezocomposite material (thisis composed of rods of lead zirconate titanate (hereinafter refferred toas PZT) set in an epoxy resin matrix), a 0-3 type piezocompositematerial (this is a piezoelectric material having piezoelectric ceramicgrains as uniformly dispersed in polymer matrix), or a 3-1 typepiezocomposite material (this is a piezoelectric material as prepared byforming a large number of through-holes in a plate of lead zirconatetitanate (PZT) followed by casting an epoxy resin or the like into thosethrough-holes and solidifying it therein).

In the invention, the wedge is made of a polyimide resin, a polystyrolresin, an acrylic resin, or a fluorine resin (Teflon).

As shown in FIG. 1, the surface wave probe 10 of the invention isessentially composed of a piezoelectric element 10A, a damping block 10Band a resin wedge 10 c. When the center frequency of the surface wavewhich the surface wave probe 10 transmits and receives is represented byfc, then the frequency bandwidth for the probe 10 is defined to be atleast 0.50 fc or larger.

Specifically, when the frequency spectrum of the surface wave which thesurface wave probe 10 transmits and receives is to have a frequencydistribution as conceptually shown in FIG. 2, the frequency bandwidthwithin which the spectrum magnitude (signal magnitude) falls the rangebetween the peak value and the peak value−6 dB and which is representedby (fR−fL) is defined to satisfy the following formula (2):

fR−fL≧0.50 fc  (2)

In the invention, as above, the frequency bandwidth for the surface waveprobe 10 is broad and is equal to or larger than 0.50 fc. The concrete,constitution of the surface wave probe 10 is described. As thepiezoelectric element 10A, usable are any piezoelectric material havinga low mechanical Q value such as lead meta-niobate, piezocompositematerial illustrated in FIGS. 23 to 25, etc., or any other piezoelectricmaterial capable of being mechanically damped with ease even thoughhaving a high mechanical Q value, such as lead titanate, etc. Themechanical Q value as referred to herein is meant to indicate thesharpness of the resonance. Piezoelectric elements having a largermechanical Q value vibrate for a longerduration when they are driven byelectric pulse. The center frequency fc of the surface wave to betransmitted and received by the probe must be chosen depending on thegrain size and the surface roughness of the material to be tested. Forexample, for rolls for rolling mills, the center frequency fc preferablyis, chosen between 1 and 4 MHz.

The damping block 10B is made of a solid substance to be prepared bymixing a fine powder having a large specific gravity, such as metallictungsten or the like, with an epoxy resin or the like followed bysolidifying the resulting mixture. The damping block 10B is attached tothe back surface of the piezoelectric element 10A, by which thedeformation of the piezoelectric element 10A is damped. The dampingblock having a larger volume fraction of the heavy powder such asmetallic tungsten or the like is to have a larger weight, and itsdamping effect on the piezoelectric element is greater, therebyresulting in that the length of the ultrasonic pulse from the probe isshortened.

The piezoelectric element 10A and the damping block 10B are soconstructed as to have the material constitution defined as above, andthe probe comprising them can produce (transmit) an ultrasonic pulse ofwhich the frequency bandwidth is at least 0.5 fc or larger and the pulselength is at most 2.5 times the wavelength of the surface wave to beproduced.

The resin wedge 10C is attached to the front surface of thepiezoelectric element 10A in such a manner that the ultrasonic wave fromthe piezoelectric element can be introduced into the structure to betested, while satisfying the following formula (3). Concretely, as shownin FIG. 1, the surface of the wedge 10C at which the piezoelectricelement 10A is attached thereto is so inclined that the normal line S2relative to the front surface of the piezoelectric element 10Aintersects the normal line S1 relative to the bottom surface of thewedge 10C that is to be contacted with the surface of the structure tobe tested via the coupling medium, at an incident angle θi between thetwo lines S1 and S2, and the front surface of the piezoelectric element10A is attached to the inclined surface of the wedge 10C. In order tokeep the short pulse waveform mentioned above, the wedge 10C itself isdesigned to have an attenuation coefficient as small as possible, forexample, it may be made of a polystyrol resin, a polyimide resin or thelike.

θi=sin⁻¹(CW/CR)  (3)

wherein CW indicates the velocity of the ultrasonic wave in the resinwedge, and

CR indicates the velocity of the surface wave traveling in the columnarstructure to be tested.

Different surface wave probes 10 having a center frequency fc=2 MHz wereprepared, for which the frequency bandwidth was varied by changing thematerial of the damping block, and checked for the relationship betweenthe pulse length and the frequency bandwidth. The data obtained areshown in FIG. 3, in which the frequency bandwidth equal to the centerfrequency fc is designated as 100%.

From FIG. 3, it is known that the probes having a frequency bandwidth ofat least 50% or larger can produce short pulses of which the pulselength is at most 2.5 times the wavelength of the surface wave to beproduced. The surface wave pulse thus produced to have a short pulselength is applied to flaw detection of rolls having innumerableremaining primary cracks K. Also in that case, combinations of smallreflectors K1 to K4 between that the difference in the distance from thesurface wave probe 10 is λ/2, λ indicating the wavelength of the surfacewave traveling in the structure being tested, always exist in thestructure, as in the case described hereinabove with reference to FIG.21.

FIG. 4 shows the waveforms as observed in ultrasonic testing of astructure, in which was used a short ultrasonic pulse having a frequencybandwidth of not smaller than 0.70 fc and a pulse length of being1.5-times the wavelength of the surface wave traveling in the structure,for testing the regions having therein small reflectors such as thosementioned above, and this corresponds to the above-mentioned FIG. 22showing the waveforms observed in conventional ultrasonic testing.

As shown in FIG. 4, it is known that the reflected waves from theremaining primary cracks K have a small amplitude since the pulse lengthapplied is short, and that even though those reflected waves overlap inphase with each other, the increase in the amplitude of the wave to beobserved is small. Accordingly, when the surface wave probe 10 of theinvention, which is so designed that it can transmit and receive asurface wave pulse having a short pulse length, is applied to flawdetection of rolls having innumerable remaining primary cracks K, theincrease in the amplitude of the reflected waves from the remainingprimary cracks K is effectively prevented.

The surface wave probes having a center frequency of 2 MHz but having avarying frequency bandwidth (or having a varying pulse length) weretested for the relationship between the height of the reflected wavesfrom the primary cracks K in a work roll for former stands in finishingtrain, and the ratio of (pulse length/wavelength of surface wave), andthe data obtained are shown in FIG. 5. In this test, the height of thereflected waves was represented with reference to the height of thereflected wave from a drilled hole that had been drilled toward theradial direction to have a diameter of 1 mm and a depth of 1 mm, and thedepth of the primary cracks K was about 0.15 mm.

In the drawing, the measured points A1, A2 and A3 are for the invention,for which the piezoelectric element 10A of the surface wave probe 10used was of a lead meta-niobate, and the damping block 10B of the probe10 was made of a mixture of an epoxy resin with a metallic tungstenpowder having a volume fraction of 80%, 60% or 40%, respectively. B1 andB2 are for comparative examples, for which the piezoelectric element 10Aused was of PZT, and the damping block 10B was made of a mixture of anepoxy resin with a metallic. tungsten powder having a volume fraction of80% or 60%, respectively. C1 and C2 are for conventional examples, forwhich the piezoelectric element 10A used was of PZT of two types thatdiffer in the mechanical Q value in some degree, and the damping block10B was not used. Except the matters specifically mentioned herein,substantially the same apparatus was used for the measurement.

From FIG. 5, it is known that the shorter pulse lengths gave reflectedwaves having a more lowered height from the primary cracks K.

Next, materials suitable to the piezoelectric element 10A and thosesuitable to the resin wedge 10C were investigated in detail. As thedamping block 10B, used. was a solid mixture of an epoxy resin and ametallic tungsten powder, as in the above. In the mixture, the volumefraction of the metallic tungsten powder to be mixed with the epoxyresin was 80%, 60%, 40% or 20%. For the piezoelectric element 10A,selected were a lead meta-niobate, a lead titanate, lead zirconatetitanate (PZT), barium ti-tanate, lithium niobate, a 1-3 typepiezocomposite material (FIG. 23), a 0-3 type piezocomposite material(FIG. 25), and a 3-1 type piezocomposite material (FIG. 24); and for theresin wedge 10C, selected were a polyimide resin, a polystyrol resin, anacrylic resin, and a fluorine resin (Teflon) Different surface waveprobes were produced in that manner, and tested to measure the frequencybandwidth and the pulse length of the surface wave to be transmitted andreceived by them. In addition, in the same manner as in the test for thedata shown in FIG. 5, those surface wave probes were further tested tomeasure the height of the reflected waves from the primary cracks K inthe same work roll for former stands in finishing train as that used inthe test for the data in FIG. 5. For this, the height of the reflectedwaves was represented with reference to the height of the reflected wavefrom a drilled hole that had been drilled toward the radial direction tohave a diameter of 1 mm and a depth of 1 mm, just in the same manner asin the test for the data in FIG. 5. The data obtained in the test inwhich the damping block 10B used had a volume fraction of the metallictungsten powder of 80% are shown in Table 2; those in which the dampingblock 10B used had a volume fraction of the metallic tungsten powder of60% are shown in Table 3; and those in which the damping block 10B usedhad a volume fraction of the metallic tungsten powder of 40% are shownin Table 4. In those Tables, the data of the probes with which thereflected waves from the primary cracks K had a height of larger than−11 dB (that is, the height of the reflected waves from the primarycracks K as seen by the use of the probes was not lower by at least 3 dBthan that as seen by the use of conventional probes) were omitted,except those of the probes having a PZT. The data of the probes having aPZT are in those Tables as comparative data. Table 5 shows the dataobtained in the test in which the damping block 10B used had a volumefraction of the metallic tungsten powder of 20%. As shown in Table 5,the height of the reflected waves from the primary cracks K was higherthan −11 dB. Referring back to Tables 2 to 4, it is understood that, inall cases, the lead meta-niobate, the lead titanate, the 1-3 typepiezocomposite material, the 0-3 type piezocomposite material and the3-1 type piezocomposite material are all usable as the piezocompositematerial 10A. It is also understood therefrom that the polyimide resin(having an attenuation coefficient at 2 MHz of 1.2×10² dB/m), thepolystyrol resin (having an attenuation coefficient at 2 MHz of 1.3×10²dB/m), the acrylic resin (having an attenuation coefficient at 2 MHz of1.8×10² dB/m), and the fluorine resin (Teflon, having an attenuationcoefficient at 2 MHz of 1.8×10² dB/m) are all usable as the resin wedge10B. Accordingly, it is known that the wedge member may have anattenuation coefficient at 2 MHz of not larger than 1.8×10² dB/m.Comparing the data in Tables 2 to 4 with those in Table 5, it isunderstood that the volume fraction of the metallic tungsten powder tobe in the damping block 10B must be at least 40% or larger.

Using a conventional surface wave probe, of which the pulse length is 5times the wavelength of the surface wave to be produced, in asubstantially same condition, we, the present inventors carried out anexperiment of ultrasonic testing of work rolls for former stands infinishing train. Through that our experiment, we confirmed that theheight of the reflected waves from the primary cracks as seen in thetesting just before the final roll grinding process was higher by 3 dBthan that as seen in the testing after the final roll grinding process.Accordingly, it is believed that, if the height of the reflected wavesfrom the primary cracks K seen in the ultrasonic testing be reduced byat least 3 dB, the number of the roll grinding repetitions could bereduced by at least one time. In this connection, the roll grinding canbe finished when the height of reflected waves from the primary crackson the ground roll are equal to or lower than predetermined thresholdvoltage.

Accordingly, as shown in FIG. 5, when the pulse length of the surfacewave probe, with which the data of the height of the reflected wavesfrom the primary cracks measured is lower by at least 3 dB than the dataCl as measured with the conventional surface wave probe whose pulselength is 5 times the wavelength of the surface wave to be produced, isat most 2.5 times the wavelength of the surface wave to be produced,then the difference between the measured data A3 and C1 is 3 dB. In thatcase, therefore, the number of the roll grinding repetitions could bereduced by at least one time.

Again referring back to FIG. 3, it may be said that the bandwidth of thesurface wave probe capable of transmitting and receiving surface wavesof which the pulse length is at most 2.5 times the wavelength of thesurface wave to be produced is suitably 50% or larger. Accordingly, itis understood that defining the frequency bandwidth for the surface waveprobe to be 0.50 fc or larger is effective for reducing any overgrindingof rolls to be caused by false detection of primary cracks of the rolls.This is the ground for defining the frequency bandwidth for the surfacewave probe to be 0.50 fc or larger in the invention.

As has been described in detail hereinabove, a specific surface probe isused in the invention, for which the frequency bandwidth is defined tobe 0.50 fc or larger and the pulse length is to be at most 2.5 times thewavelength of the surface wave to be produced. Comparing the data asmeasured by the use of the conventional surface wave probe of which thepulse length is about 5 times the wavelength of the surface wave to beproduced (in FIG. 5, C1 point), and those as measured by the use of thespecific surface wave probe of the invention (in FIG. 5, A1 to A3points), it is known that the height of the reflected waves from primarycracks K as measured by the use of the specific surface wave probe ofthe invention is lower by from 3 to 6 dB than that as measured by theuse of the conventional surface wave probe.

In the same manner as above, we, the inventors further carried out anexperiment for detecting cracks having a depth of 0.5 mm in rolls inwhich primary cracks K having a depth of about 0.10 mm remain and thosein high-speed tool steel rolls in which primary cracks K having a depthof about 0.25 mm remain. Through that our experiment, we found that theS/N ratio of the reflected waves from the cracks as measured by the useof a surface wave probe of which the pulse length was 1.5 times thewavelength of the surface wave produced was 10 dB and the S/N ratio ofthe reflected waves from the cracks as measured by the use of a surfacewave probe of which the pulse length was 2.5 times the wavelength ofsurface wave produced was 7 dB, while, on the other hand, the S/N ratioof the reflected waves from the cracks as measured by the use of aconventional surface wave probe of which the pulse length was about 5times the wavelength of the surface wave produced was about 4 dB. By useof the method of the invention, therefore, the signal-to-noise ratio ofthe reflected waves from cracks could be higher by about 3 to 6 dB thanthat by use of the conventional method, and the detectability in theinvention are significantly enhanced.

Next, using an ultrasonic testing apparatus equipped with the surfacewave probe 10 illustrated in FIG. 1, in which the piezoelectric element10A, the damping block 10B and the resin wedge 10C are made of a leadmeta-niobate, a mixture of an epoxy resin with a metallic tungstenpowder having a volume fraction of 60%, and a polyimide resin,respectively. We, the inventors further carried out an experiment ofinspecting 500 work rolls for former stands in finishing train.Specifically, in the experiment, we measured the decrement in diameterof each roll until the height of the reflected waves from the primarycracks (fire cracks) in each roll reached a predetermined level orlower. Using another ultrasonic test apparatus equipped with aconventional surface wave probe of which the pulse length is about 5times the wavelength of the surface wave to be produced, we carried outthe same experiment. As a result, the decrement in diameter of rolls bygrinding by use of the apparatus equipped with the conventional surfacewave probe was 0.33 mm on the average, while the decrement in diameterof rolls by grinding by use of the apparatus equipped with the surfacewave probe of the invention was 0.2 mm on the average.

As in this experiment, the decrement in diameter of rolls by grinding onthe basis of the technique of the invention is lowered by at least 0.1mm on the average, as compared with that on the basis of theconventional technique. In this connection, we confirmed that when theroll having been ground in that manner on the basis of the technique ofthe invention was used in rolling sheets, the degree of the surfaceflaws that appeared in the rolled sheets owing to the small pits formedon the surface of the roll in use was substantially the same as that inthe sheets having been rolled by the use of the roll as tested andground on the basis of the conventional technique.

The invention also provides an ultrasonic test apparatus for detectingflaws in columnar structures, in which a surface wave probe is contactedwith the surface of a rotating columnar structure via a coupling liquidmedium existing therebetween, and a surface wave is propagated into thecolumnar structure from the surface wave probe while the probe receivesthe reflected waves from the flaws existing in and just below thesurface of the columnar structure so as to detect the flaws. Theapparatus comprises a columnar structure-rotating means for rotating thecolumnar structure in the circumferential direction of the structure; arotating speed-monitoring means for monitoring the rotating speed of thecolumnar structure being rotated by the columnar structure-rotatingmeans; a holder means for holding the surface wave probe above thecolumnar structure to ensure a predetermined distance between the probeand the surface of the columnar structure; a scanning means for scanningthe probe in the axial direction of the structure; a couplant supplymeans capable of supplying a liquid medium to be a coupling medium forultrasonic waves to the gap between the surface wave probe and thesurface of the columnar structure and provided with a flow control valvecapable of controlling the flow rate of the liquid medium in accordancewith the rotating speed of the columnar structure to be rotated by thecolumnar structure-rotating means; a surface wave probe which isprovided with a piezoelectric element, a wedge disposed on the frontsurface of the piezoelectric element and a damping block disposed on theback surface of the piezoelectric element, so that, where the centerfrequency of the surface wave to be transmitted and received by theprobe is fc in the frequency spectrum, the frequency bandwidth withinwhich the spectrum magnitude falls the range between the peak value andthe peak value−6 dB covers at least 0.50 fc or larger, and that theprobe is capable of detecting the flaws in the columnar structure usingsurface waves; an ultrasonic pulser/receiver capable of supplying to thesurface wave probe, an electric pulse for producing a surface waves andcapable of amplifying the signals which the surface wave probe hasreceived to a level necessary for flaw detection and outputting it; agating means for extracting the signals for flaw detection from thesignals which the ultrasonic pulser/receiver has outputted, andoutputting them; and a peak detector/comparator means for detecting theamplitude of the signals which the gating means has outputted, andoutputting the thus-detected signals, or for comparing the level of thesignals which the gating means has outputted with a predeterminedthreshold voltage and, when the level of the thus-compared signals islarge, outputting signals that indicate the presence of flaws in thestructure being tested. The apparatus of the invention has attained theabove-mentioned third subject matter.

In particular, in the apparatus noted above where one surface of thewedge of the probe is so inclined that the normal line relative to theinclined surface intersects the normal line relative to the bottomsurface of the wedge to be contacted with the surface of the columnarstructure to be tested via a coupling medium existing therebetween at anincident angle θi to be defined by the above-mentioned formula (3) andwhere the front surface of the piezoelectric element of the probe isattached to the thus-inclined surface of the wedge, the surface waveshaving been transmitted by the probe can be well propagated into thesurface of the columnar structure.

We, the present inventors further carried out still another testexperiment using a high-speed tool steel roll having artificial flawstherein. In the experiment, the rotating speed of the roll was varied,and a varying amount of a coupling liquid medium (as the medium, waterwas used in the experiment) was applied to the rotating roll in order todetermine the suitable amount of the coupling medium via which theultrasonic wave having been produced by the probe could be stablytransmitted into the roll surface without any surplus liquid mediumflowing ahead of the probe, and the height of the reflected waves fromthe artificial flaws could be kept constant. The data obtained in theexperiment are shown in FIG. 6. Where the amount of the medium (water)falls within the range as shadowed in the graph of FIG. 6, theultrasonic waves from the probe are stably transmitted to the rollsurface. From the data obtained, it is known that, with the increase inthe rotating speed of the high-speed tool steel roll being tested, theamount of the liquid medium to be supplied to the rotating roll must beincreased. Accordingly, when the rotating speed of the high-speed toolsteel roll being tested is monitored by means of a rotating speedmonitor and the amount of the liquid medium to be supplied to the rollis controlled in accordance with the rotating speed of the roll by meansof the flow control valve as provided to the couplant supply means, thena suitable amount of the coupling liquid medium can be supplied to thegap between the probe and the surface of the roll. In that manner, thetransmission of the ultrasonic wave from the probe into the roll surfaceis stabilized, and any surplus liquid medium is prevented from flowingahead of the probe.

To attain the above-mentioned third subject matter, the inventionfurther provides a method of grinding a roll of which the surface hasbeen thermally/mechanically damaged. In the invention, a surface waveprobe is contacted with a roll to be ground or being ground, via amembrane of a coupling medium existing therebetween, while the roll isrotated, so that surface waves from the probe is propagated into theroll surface while removing the liquid from the path of surface waves,and the height of the reflected waves from the thermally/mechanicallydamaged parts existing or remaining in the surface of the roll ismeasured, and the grinding allowance of the roll is determined inaccordance with the thus-measured height of the reflected waves.

The surface wave probe for roll testing in the invention is contactedwith the surface of a rotating roll via a coupling medium existingtherebetween, and this comprises at least a piezoelectric element and awedge that introduces the ultrasonic wave from the piezoelectric elementinto the roll surface at an incident angle θi. In the invention, theprobe is so disposed that it produces the surface waves into the rollsurface and detects the flaws existing in and just below the rollsurface using the thus-produced surface waves. In this, the incidentangle θi is defined to satisfy the following formula (4), by which theabove-mentioned subject matter is attained.

θi=sin⁻¹(CW/CRav)  (4)

wherein CW indicates the velocity of the ultrasonic wave in the wedge,and

CRav indicates the mean value of the velocity of the surface wavetraveling in each roll to be tested.

The incident angle θi is an angle to the plane vertical to the rollsurface.

To attain the above-mentioned subject matter, the invention stillfurther provides a method of defining the incident angle for a surfacewave probe for roll testing, in which the probe is contacted with thesurface of a rotating roll via a coupling medium existing therebetween,and this comprises at least a piezoelectric element and a wedge thatintroduces the ultrasonic wave from the piezoelectric element into theroll surface at an incident angle θi, while being so disposed that itproduces the surface waves into the roll surface and detects the flawsexisting in and just below the roll surface using the thus-producedsurface waves, and in which the incident angle θi is defined to satisfythe above-mentioned formula (4).

Using the surface wave probe, we, the present inventors carried outstill another experiment to know the relationship between the height ofthe reflected waves from the thermally/mechanically damaged parts of aroll and the layer thickness of the still remaining,thermally/mechanically damaged parts of the roll (in the experiment,when the height of the reflected waves measured is lower than thethreshold voltage for flaw detection, we say that the remaining layerthickness of the damaged part is zero). In the experiment, we groundrolls that had been thermally/mechanically damaged in rolling operation,little by little, while measuring the height of the reflected waves fromthe thermally/mechanically damaged parts of each roll to know therelationship noted above. The data we obtained are in FIG. 7, from whichit is well known that, with the decrease in the remaining layerthickness of the thermally/mechanically damaged parts of the rolltested, the height of the reflected waves from thethermally/mechanically damaged parts lowers. From the data in FIG. 7,obtained was the relationship between the grinding allowance of the rollfor removing the thermally/mechanically damaged parts from the roll, andthe height of the reflected waves from the thermally/mechanicallydamaged parts. This is as shown in FIG. 8. Accordingly, before or duringgrinding rolls, the height of the reflected waves from thethermally/mechanically damaged parts of each roll is measured by surfacewave testing, and the grinding allowance of the roll may be determinedaccording to the relationship as plotted in FIG. 8. In that manner, anyovergrinding of the part not mechanically damaged and therefore needlessto be removed, and any grinding failure to completely remove thethermally damaged parts can be prevented, and optimum grinding of rollsis possible.

In one preferred embodiment, the surface wave probe and the grindstoneare moved to the position of the roll to be ground, at which the heightof the reflected waves from the thermally/mechanically damaged parts ofthe roll is the largest, and the roll is ground by means of plungegrinding while being tested by surface waves. For this, the decrement indiameter of the roll by grinding until the height of the reflected wavesfrom the thermally/mechanically damaged parts reaches a predeterminedlevel or lower is measured and the grinding allowance is determined fromthe measured decrement. After that the roll is further ground inaccordance with the thus-determined grinding allowance. In that manner,the optimum grinding of the roll is realized.

Specifically, before or during grinding them, rolls are subjected to thesurface wave testing in the manner as above. Through this testing it ispossible to identify the position of each roll at which the height ofthe reflected waves from the thermally/mechanically damaged parts is thelargest, as the position having a largest layer thickness of theremaining thermally/mechanically damaged parts, as is obvious from therelationship as plotted in FIG. 7. Having known this, a roll may beground as shown in FIG. 9, in which the grindstone 62 of a roll grinderand the surface wave probe 10 are located to the roll 110 at the sameposition (relative to the axial direction of the roll) where the heightof the reflected waves from the thermally/mechanically damaged parts isthe largest. In that condition, the roll 110 is ground by means ofplunge grinding until the height of the reflected waves from thethermally/mechanically damaged parts of the roll reaches a predeterminedthreshold voltage or lower, while being subjected to surface wavetesting, and the decrement in diameter of the roll to be thus ground isthe grinding allowance for removing the thermally/mechanically damagedparts from the entire surface of the roll. According to thethus-determined grinding allowance, the entire surface of the roll maybe ground.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged, cross-sectional view showing the outline of thestructure of a surface wave probe to be used in the invention.

FIG. 2 is a graph for explaining the frequency bandwidth for the surfacewave probe.

FIG. 3 is a graph showing the relationship between the frequencybandwidth for the surface wave probe and the ratio of (pulselength/wavelength of surface wave).

FIG. 4 is an explanatory view indicating the relationship between thewaveform to be observed on the basis of the invention, and the reflectedwaves from small reflectors.

FIG. 5 is a graph showing the relationship between the height of thereflected waves from primary cracks and the ratio of (pulselength/wavelength of surface wave).

FIG. 6 is a graph showing the relationship between the rotating speed ofa roll and a preferred amount of the coupling liquid medium to besupplied to the gap between the probe and the roll, which is forexplaining the principle of the invention.

FIG. 7 is a graph showing the relationship between the height of thereflected waves from thermally/mechanically damaged parts of a roll andthe remaining layer thickness of the thermally/mechanically damagedparts, which is also for explaining the principle of the invention.

FIG. 8 is a graph showing the relationship between the grindingallowance of a roll and the height of the reflected waves fromthermally/mechanically damaged parts of the roll, which is also forexplaining the principle of the invention.

FIG. 9 is a perspective view showing the relationship between theposition of the grindstone and that of the surface wave probe in plungegrinding of a roll, which is also for explaining the principle of theinvention.

FIG. 10 is a side view showing the outline of the constitution of thefirst embodiment of the ultrasonic test apparatus of the invention.

FIG. 11 is an enlarged front view showing the probe holder part in thefirst embodiment.

FIG. 12 is a partly-cleaved side view showing the major part of thecoupling medium supply part as fitted to the surface wave probe in thefirst embodiment.

FIG. 13 is a side view showing the outline of the constitution of thesecond embodiment of the ultrasonic test apparatus of the invention.

FIG. 14 is a graph showing the data of an experiment for the secondembodiment relative to the relationship between the height of thereflected waves from the flaws and the rotating speed of a roll, forwhich the rotating speed of the roll was varied for flaw detectionaccording to the second embodiment.

FIG. 15 is a side view showing the outline of the constitution of thethird embodiment of the ultrasonic test apparatus of the invention.

FIG. 16 is a side view showing the outline of the constitution of thefourth embodiment of the ultrasonic test apparatus of the invention.

FIG. 17 is a graph showing the data of the height of the reflected wavesfrom artificial flaws in five rolls, for which were used a probe F ofthe invention and conventional probes G, H.

FIG. 18 is a graph showing the data of the signal-to-noise ratio for thereflected waves from artificial flaws in five rolls, for which were usedthe probe F of the invention and the conventional probes G, H.

FIG. 19 is a graph showing the data of roll testing for 20 surfaceflaws, in terms of the signal-to-noise ratio, for which were used theprobe F of the invention and the conventional probes G. H.

FIG. 20 is a conceptual view for explaining the cracks to be formed inthe surface of a work roll for former stands in finishing train, in thecircumferential direction of the roll.

FIG. 21 is an explanatory view showing the relationship between theposition of a surface wave probe and that of small reflectors.

FIG. 22 is an explanatory view indicating the relationship between thewaveform to be observed in a conventional method, and the reflectedwaves from small reflectors.

FIG. 23 shows one example of the piezoelectric element for the invention(this is a 1-3 type piezocomposite material).

FIG. 24 shows another example of the piezoelectric element for theinvention (this is a 3-1 type piezocomposite material).

FIG. 25 shows still another example of they piezoelectric element forthe invention (this is a 0-3 type piezocomposite material).

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the invention are described in detail hereinunder, withreference to the drawings.

FIG. 10 is a side view showing the outline of the constitution of thefirst embodiment of the ultrasonic test apparatus of the invention.

The ultrasonic test apparatus of this embodiment is for testing the roll110, and its basic constitution comprises a roll rotating device forrotating the roll 110, a surface wave probe 10 for transmitting andreceiving the surface waves in the roll 110, a probe holder 12 forholding the probe 10, and a couplant supply means for supplying acoupling liquid medium (water) to the gap between the surface of theroll 110 and the surface wave probe 10. The supply means is connectedwith the holder 12, and this will be described hereinunder.

For simplifying the drawing, the roll rotating device is not showntherein. This may be any known one, provided that it acts to rotate theroll 110 to be tested in the circumferential direction C of the roll.

For the surface wave probe 10, the type of the piezoelectric element andthe composition of the damping block are so designed that the probe 10can transmit and receive a surface wave having a frequency bandwidth ofat least 0.50 fc or larger and having a pulse length of at most 2.5times the wavelength of the surface wave.

The surface wave probe 10 is so disposed relative to the roll 110 to betested that the gap between the probe 10 and the roll 110 could befilled with water (coupling liquid medium). In that condition, anultrasonic wave is transmitted into the surface of the roll 110 viawater to produce surface waves that is propagated into the surface ofthe roll 110, and the probe 10 receives the reflected waves to detectthe surface flaws in the roll 110.

The probe holder 12 holds the surface wave probe 10, and is supported bythe supporting member 18 as fitted to the lower part of the guide 16,and the guide 16 is slidable up and down relative to the fixing member14 as positioned above the roll 110. The supporting member 18 isprovided with a pair of rollers 20 at its front and back, and therollers 20 total four. Between those rollers 20, disposed is the probeholder 12. When the apparatus is operated for roll testing, these fourrollers 20 are rotated while being kept in contact with the surface ofthe roll 110 so as to stabilize the test scanning.

The fixing member 14 is provided with a motor 14A which is to supplypower for sliding up and down the supporting member 18 along the guide16, through a known mechanical means (not shown), and with a fittingbase 14B for the motor 14A.

The fixing member 14 can be scanned in the axial direction of the roll110 by a scanning means (not shown), whereby the surface wave probe 10can be scanned in the axial direction of the roll 110.

The probe holder 12 is fitted to the lower end of the rod member 12A,and the rod member 12A is loosely clamped to the supporting member 18 sothat it is movable up and down relative to the member 18. In thatcondition, the probe holder 12 is supported by the supporting member 18while it is all the time pressed downward in the drawing, or that is,against the surface of the roll 110 by means of springs (not shown)provided at predetermined positions around the rod member 12A.

The probe holder 12 is provided with a pair of following rollers 22which are for forming a predetermined gap between the surface wave probe10 and the roll 110 and which protrude from beneath the surface waveprobe 10 toward the roll 110.

FIG. 11 is an enlarged front view showing the condition, in which shafts24 are disposed at the both opposite sides of the probe holder 12 in thehorizontal direction (that is, in the axial direction of the roll 110),and each of the above-mentioned following rollers 22 is rotatably fittedto each shaft 24. In that manner, the following rollers 22 as pivoted onthe probe holder 12 receive suitable pressing force from theabove-mentioned springs and are all the time kept in contact with thesurface of the roll 110 while the roll 110 is tested. According to theconstitution of the probe holder 12 noted above, the surface wave probe10 is held above the roll 110 in such a manner that a predetermined gapis ensured all the time between the probe 10 and the roll 110.

As shown in FIG. 12 in which the details of the probe holder 12 and thefollowing rollers 22 are omitted but their outlines are designated bytwo-dotted lines, the probe holder 12 is provided with a water supply(water-supplying means) 26 existing inside it. Water having been ledthrough the duct 28 is once stored in the storing body 26A and is letout through the outlet hole 26B formed at the bottom of the storing body26A. In that manner, a non-bubbling water layer is formed between thesurface wave probe 10 and the roll 110. The water supply may be anyknown conventional one, and the detailed description of its structure isomitted herein.

In FIG. 10, the numeral reference 30 indicates a scraper for scrapingwater so that water having been supplied from the water supply 26 in themanner noted above is prevented from remaining on the surface of theroll and flowing into path of the surface waves on the roll.

As being designed in the manner described in detail hereinabove, theultrasonic test apparatus of this embodiment ensures the testing ofrolls in a simple manner while water to be a coupling medium forultrasonic wave propagation through the gap between the surface waveprobe 10 and the surface of the roll 110 to be tested and while theprobe 10 is scanned and moved on the surface of the roll 110.

According to this embodiment, 500 work rolls (high-speed tool steelrolls) for former stands in finishing train were tested, and thedecrement in diameter of each roll by grinding until the height of thereflected waves from primary cracks which are so-called fire cracksreaches a predetermined level or lower was measured. Using anotherultrasonic test apparatus equipped with a conventional surface waveprobe of which the pulse length is about 5 times the wavelength of thesurface wave to be produced, the same rolls were also tested in the samemanner. As a result, the decrement in diameter, of each roll by grindingby means of the apparatus equipped with the conventional surface waveprobe was 0.33 mm on the average, while the decrement in diameter ofeach roll by grinding by means of the apparatus of this embodiment was0.2 mm on the average. As in this experiment, the decrement in diameterof rolls by grinding based on the technique of the invention is loweredby at least 0.1 mm, as compared with that based on the conventionaltechnique. In this connection, it was confirmed that when the high-speedtool steel roll having been ground in that manner on the basis of thetechnique of the invention was used in rolling sheets, the degree of thesurface defects that appeared in the rolled sheets owing to the smallpits formed on the surface of the roll in use was substantially the sameas that in the sheets having been rolled by the use of the roll astested and ground on the basis of the conventional technique.

Referring to FIG. 13, the second embodiment of the invention that issuitable for testing high-speed tool steel rolls by scanning the surfacewave probe 10 on the surface of each roll is described in detailhereinunder.

In this embodiment, the roll 110 is a high-speed tool steel roll, andthe rotating speed of the roll is, monitored by the rotating speedmonitor 32. The data thus monitored are transmitted to the flow controlvalve 34 connected with the water supply, and the water flow rate is socontrolled as to be within the preferred range as in FIG. 6. The signalsoutputted by the surface wave probe 10 are inputted into the peakdetector/comparator circuit 44 processed by the ultrasonicpulser/receiver 40 and the gating circuit 42 in that order.

The ultrasonic pulser/receiver 40 is to supply an electric pulse to thesurface wave probe 10 for producing the surface waves. In thispulser/receiver 40, the signals having been received by the surface waveprobe 10 is amplified to a level necessary for flaw detection, and areoutputted to the gating circuit 42. In the gating circuit 42, thesignals for flaw detection are extracted from the signals having beenoutputted from the ultrasonic pulser/receiver 40, and are outputted tothe peak detector/comparator circuit 44. In the peak detector/comparatorcircuit 44, the peak amplitude of the signals having been outputted fromthe gating circuit 42 is detected, and the thus-detected signal isoutputted from it; or in this, the level of the signals from the gatingcircuit 42 is compared with a predetermined threshold voltage and, whenthe level of the thus-compared signal from the gating circuit 42 islarge, signals that indicate the presence of flaws in the roll beingtested is outputted from the peak detector/comparator circuit 44 Beingoperated in that manner, the apparatus of this second embodiment detectsthe flaws in rolls being tested.

Like that in the first embodiment, the probe holder 12 in the secondembodiment is provided with a water supply 26 in its inside as shown inFIG. 12. The flow rate of water from the water supply 26 is controlledby the flow control valve 34 according to the rotating speed (peripheralspeed) of the roll 110. In the water supply 26, water having been ledthrough the duct 28 is once stored in the storing body 26A and is letout through the outlet hole 26B formed at the bottom of the storing body26A. In that manner, a non-bubbling water layer is formed between thesurface wave probe 10 and the roll 110.

The others in this embodiment are the same as those in the firstembodiment noted above and are designated by the same reference numeralsas in the first embodiment. The detailed description of these others isomitted herein.

High-speed tool steel rolls having surface flaws were tested by use ofthe apparatus of this embodiment, while their rotating speed was variedfrom 25 rpm to 50 rpm. The data obtained for the relationship betweenthe height of the reflected waves from the flaws and the rotating speedof each roll are shown in FIG. 14. From these, it is known that theapparatus of this embodiment well detect the surface flaws in thehigh-speed tool steel rolls tested, irrespective of the rotating speedof the rolls.

Next, referring to FIG. 15, the third embodiment of the invention isdescribed in detail hereinunder.

In this embodiment, rolls to be ground or being ground are tested bymeans of surface waves. Specifically, in the apparatus of thisembodiment, the height of the reflected waves fromthermally/mechanically damaged parts of the roll 110 is measured, andthe grinding allowance for the roll 110 to be ground is transmitted tothe grinder 60 in which the roll 110 is ground. The grinder may be anyknown conventional one, and is not shown for simplifying the drawing.

The surface wave probe 10 is connected with the ultrasonic flaw detector50, and an electric pulse is supplied to the surface wave probe 10 fromwhich the surface wave produced is transmitted into the roll 110. In theultrasonic flaw detector 50, the signals which the surface wave probe 10has received and outputted to the ultrasonic flaw detector 50 areamplified to a level suitable to flaw detection. The ultrasonicultrasonic flaw detector 50 is provided with a gating circuit (no shown)that may be the same as in the second embodiment, and the reflectedwaves from the thermally/mechanically damaged parts in the roll 110 areextracted from the amplified signals in the gating circuit. In theultrasonic flaw detector 50, the height of the thus-extracted, reflectedwaves is measured. Having been thus measured in the ultrasonic flawdetector 50, the data of the height of the reflected waves from thethermally/mechanically damaged parts are transmitted to the computer 52,in which the grinding allowance for removing the thermally/mechanicallydamaged parts is determined with reference to the relationship as shownin FIG. 8. The thus-settled data of the grinding allowance aretransmitted to the grinder 60, in which rolls are ground with, forexample, a grindstone.

The others in this embodiment are the same as those in the first andsecond embodiments noted above and are designated by the same referencenumerals as in them. The detailed description of these others is omittedherein.

According to this embodiment, 200 work rolls for former stands infinishing train were tested, and the decrement in diameter of each rollby grinding was measured. Apart from this, the decrements in diameter ofthe same rolls by grinding based on a conventional method were presumedfrom the actual decrements measured. In a conventional method, the rollsare repeatedly ground by a predetermined grinding allowance until theheight of the reflected waves from the thermally/mechanically damagedparts becomes lower than a predetermined threshold voltage in thesurface wave testing after ground. The above-mentioned actual decrementwas measured after the grinding based on the method using the apparatusof this embodiment of the invention. The presumed decrement in diameterin the conventional method was 0.23 mm on the average. As opposed tothis, the decrement in diameter by grinding based on the method of theinvention for suitable grinding of rolls was 0.18 mm on the average.This means that the decrement in diameter by grinding based on themethod using the apparatus of this embodiment is lower by at least 0.05mm than that based on the conventional method.

Next, referring to FIG. 16, the fourth embodiment of the presentinvention is described in detail hereinunder.

In this embodiment, provided is a position monitor 36 for monitoring theposition of the surface wave probe 10 relative to the axial direction ofthe roll being tested. The data of the position of the surface waveprobe 10 having been monitored by the position monitor 36 aretransmitted to the computer 52. In surface wave testing of rolls to beground or being ground according to this embodiment, the position of thesurface wave probe 10 that is in contact with the specific place of theroll at which the height of the reflected waves from thethermally/mechanically damaged parts is the largest is determined by theposition monitor 36. For so-called plunge grinding based on thisembodiment, as shown in FIG. 9, the surface wave probe 10 and thegrindstone 62 are mechanically so aligned that the two are to becontacted with the roll 110 at the same position relative to the axialdirection of the roll 110.

The others in this embodiment are the same as those in the thirdembodiment noted above and are designated by the same reference numeralsas in the third embodiment. The detailed description of these others isomitted herein.

The operation of this embodiment is described in detail. First, whilethe roll 110 to be ground or being ground is rotated in itscircumferential direction C, the surface wave probe 10 is scanned overthe roll 110 in the axial direction of the roll 110. In that manner, theentire surface of the roll 110 is tested by use of the surface wavestraveling thereon, and the height of the reflected waves from thethermally/mechanically damaged parts of the roll 110 and also the signalthat indicates the position of the surface wave probe 10 are inputtedinto the computer 52. By the action of the computer 52, the position ofthe surface wave probe 10 that is in contact with the specific place ofthe roll 110 at which the height of the reflected waves from thethermally/mechanically damaged parts is the largest is determined.

Next, as shown in FIG. 9, the surface wave probe 10 and the grindstone62 are moved to the thus-determined position of the roll 110, and theroll 110 is ground by means of plunge grinding while being subjected tosurface wave testing. The grinding is continued until the height of thereflected waves from the thermally/mechanically damaged parts becomeslower than a predetermined threshold voltage, and the grinding allowanceof the roll is thus determined.

The thus-determined grinding allowance is inputted into the grinder 60,in which the remaining surface area of the roll is then ground.

According to this embodiment, 200 work rolls for former stands infinishing train were tested, and the decrement in diameter of each rollby grinding was measured. Apart from this, the decrements in diameter ofthe same rolls by grinding based on a conventional method were presumedfrom the actual decrements measured. In a conventional method where therolls are repeatedly ground by a predetermined grinding allowance untilthe height of the reflected waves from the thermally/mechanicallydamaged parts becomes lower than a predetermined threshold voltage inthe surface wave testing after ground. The presumed decrement indiameter by grinding based on the conventional method was 0.24 mm on theaverage. As opposed to this, the decrement in diameter by grinding basedon the method of the invention for suitable grinding of rolls was 0.19mm on the average. This means that, the decrement in diameter bygrinding the method using the apparatus of this embodiment is lower byat least 0.05 mm than that based on the conventional method.

Next, referring to FIG. 17 and FIG. 18, the fifth embodiment of thepresent invention is described in detail hereinunder.

FIG. 17 and FIG. 18 are graphs showing the measuring results of thereflected waves from artificial flaws in five rolls shown in Table 1,for which were used a surface wave probe of the invention and twoconventional surface wave probes. FIG. 17 shows the height of thereflected waves from the artificial flaws in those rolls; and FIG. 18shows the signal-to-noise ratio of the reflected waves from theartificial flaws in those rolls. The flaws were artificially made bydrilling each roll toward radial direction to have a diameter of 1 mmand a depth of 1 mm. The wedge of the surface wave probe used herein wasof a polystyrol resin (CW=2340 m/sec).

The following three surface wave probes were prepared and used.

Probe F:

For this, the data of the surface wave velocity on the five rolls to betested were averaged to obtain an average value, CRav. From the valueCRav and the ultrasonic wave velocity in the polystyrol resin, CW,obtained was θi according to the formula (2) mentioned above. θi was48.1 degrees. A surface wave probe was so designed as to meet θi=48.1degrees. This is Probe F, and this falls within the scope of theinvention.

Probe G:

A surface wave probe was so designed as to meet θi=49.2 degrees, whichwas calculated from the surface wave velocity on Roll #4 in Table 1 andthe ultrasonic wave velocity in the polystyrol resin. This is Probe G,and this is a conventional surface wave probe.

Probe H:

A surface wave probe was so designed as to meet θi=51.7 degrees, whichwas calculated from the surface wave velocity on ordinary steel (2980m/sec) and the ultrasonic wave velocity in the polystyrol resin. This isProbe H, and this is another conventional surface wave probe.

From FIG. 17 and FIG. 18, it has been verified that Probe F of theinvention gives high reflected waves and stable S/N ratios irrespectiveof the type of rolls to be tested therewith. In FIG. 17, the verticalaxis indicates the height of the reflected waves from the artificialflaws with reference to the height of the reflected wave from theartificial flaw on Roll #1 detected by use of Probe F.

Next, four actual surface flaws for each of five roll material in Table1, totaling 20 surface defects in all rolls, were sampled, and the S/Nratios of the reflected waves from those surface flaws were measured.FIG. 19 is a graph of the thus-measured data, in which the horizontalaxis indicates the serial numbers of those 20 surface flaws, and thevertical axis indicates the signal-to-noise ratio of reflected wave fromeach surface flaw. In this experiment, used were the above-mentionedthree probes, Probe F, Probe G and Probe H. From FIG. 19, it has beenverified that Probe F of the invention can detect the actual surfaceflaws in a stable manner at high S/N ratios.

The invention has been described concretely hereinabove. However, theinvention is not limited to only the above-mentioned embodiments but canbe changed and modified in different manners without overstepping thespirit and scope thereof.

For example, the materials of the piezoelectric element 10A, theclamping block 10B and the resin wedge 10C that constitute the surfacewave probe are not limited to only those shown in the above-mentionedembodiments, and any other materials having the same functions areusable herein.

In the above-mentioned embodiments, water is used as the couplingmedium. Apart from this, any other liquids such as oils, etc. may beused herein.

The subjects to which the invention is applied are not limited to onlyrolls for rolling mills, especially to high-speed tool steel rolls, butinclude any columnar structures such as rollers of metals and otherswith no specific limitation.

TABLE 1 Roll No. Surface Wave (Roll Production Velocity Material)Manufacturer Method (m/sec.) #1 A Company continuous 3158 casting withbuild-up surfacing #2 A Company centrifugal 3110 casting #3 B Companycentrifugal 3168 casting #4 C Company centrifugal 3090 casting #5 DCompany forging 3180

TABLE 2 volume fraction of metallic tungsten powder: 80 % Height ofReflected Frequency Bandwidth Pulse Waves from Primary PiezoelectricMaterial Resin Material (%) Length/Wavelength Cracks (dB) Remarks LeadMeta-niobate Polyimide 75 1.5 −13.5 Examples of the Polystyrol 75 1.5−13.4 Invention Acryl 70 1.5 −12.8 Teflon 70 1.5 −12.5 Lead TitanatePolyimide 62 2 −12 Polystyrol 63 2 −12.4 Acryl 60 2 −11.6 Teflon 59 2−11.8 1-3 Type Polyimide 75 1.5 −13.2 Piezocomposite Polystyrol 73 1.5−13.4 Acryl 70 1.5 −12.4 Teflon 69 1.5 −12.6 0-3 Type Polyimide 61 2 −12Piezocomposite Polystyrol 61 2 −11.8 Acryl 58 2 −11.6 Teflon 58 2 −11.83-1 Type Polyimide 62 2 −11.6 Piezocomposite Polystyrol 62 2 −11.4 Acryl60 2 −11.6 Teflon 59 2 −11.8 PZT Polyimide 38 3.5 −9.6 ComparativeExamples Polystyrol 38 3.5 −9.4 Acryl 37 3.5 −9.2 Teflon 36 3.5 −9

TABLE 3 volume fraction of metallic tungsten powder: 60 % Height ofReflected Frequency Bandwidth Pulse Waves from Primary PiezoelectricMaterial Resin Material (%) Length/Wavelength Cracks (dB) Remarks LeadMeta-niobate Polyimide 62 2 −12.4 Examples of the Polystyrol 62 2 −12.3Invention Acryl 60 2 −12.1 Teflon 60 2 −12.1 Lead Titanate Polyimide 522.5 −11.1 Polystyrol 52 2.5 −11 Acryl 50 2.5 −11 Teflon 50 2.5 −11.1 1-3Type Polyimide 61 2 −12.4 Piezocomposite Polystyrol 60 2 −12.2 Acryl 602 −12 Teflon 60 2 −12.1 0-3 Type Polyimide 51 2.5 −11.2 PiezocompositePolystyrol 51 2.5 −11.2 Acryl 50 2.5 −11 Teflon 50 2.5 −11 3-1 TypePolyimide 52 2.5 −11.3 Piezocomposite Polystyrol 50 2.5 −11 Acryl 50 2.5−11 Teflon 50 2.5 −11 PZT Polyimide 32 4 −9 Comparative ExamplesPolystyrol 33 4 −8.8 Acryl 31 4 −8.9 Teflon 31 4 −8.7

TABLE 4 volume fraction of metallic tungsten powder: 40% Height ofReflected Frequency Bandwidth Pulse Waves from Primary PiezoelectricMaterial Resin Material (%) Length/Wavelength Cracks (dB) Remarks LeadMetal-niobate Polyimide 52 2.5 −11.4 Examples of the Polystyrol 52 2.5−11.3 Invention Acryl 50 2.5 −11.1 Teflon 50 2.5 −11.1 1-3 TypePolyimide 51 2.5 −11.1 Piezocomposite Polystyrol 51 2.5 −11 Acryl 50 2.5−11.1 Teflon 50 2.5 −11 PZT Polyimide 27 5 −8.4 Comparative ExamplesPolystyrol 26 5 −8.6 Acryl 26 5 −8.4 Teflon 26 5 −8.5

TABLE 5 volume fraction of metallic tungsten powder: 20% Height ofReflected Frequency Bandwidth Pulse Waves from Primary PiezoelectricMaterial Resin Material (%) Length/Wavelength Cracks (dB) Remarks LeadMetal-niobate Polyimide 52 2.5 −10.6 Comparative Examples Polystyrol 522.5 −10.4 Acryl 50 2.5 −10.3 Teflon 50 2.5 −10.4 1-3 Type Polyimide 512.5 −10.5 Piezocomposite Polystyrol 51 2.5 −10.4 Acryl 50 2.5 −10.3Teflon 50 2.5 −10.2 PZT Polyimide 22 6 −8.2 Polystyrol 21 6 −8 Acryl 206 −8.1 Teflon 20 6 −8.1

INDUSTRIAL APPLICABILITY

According to the present invention for surface wave testing, falsedetection of primary cracks is prevented and overgrinding that causesthe increase in the roll consumption is prevented. In addition, thelevel of structual noises from primary cracks and grain boundaries islowered, and the detectability of the apparatus are greatly enhanced.

In particular, in the process on the basis of the invention where rollshaving been thermally/mechanically damaged in their surfaces while theyare used in rolling are ground and the grinding allowance of each rollis settled according to the height of the reflected waves from thethermally/mechanically damaged parts, the decrement in diameter of eachroll by grinding is optimized to reduce the roll consumption and toimprove the roll grinding efficiency.

In applications of surface wave testing of rolls on the basis of theinvention, the surface wave probe can produce and receive the surfacewave at high efficiency, irrespective of the difference in rollmaterials as produced in different methods. In those, therefore, one andthe same surface wave probe is usable in testing of rolls of differentmaterials without exchanging it, and the surface wave probe common tosuch different rolls ensures increased signal-to-noise ratios for thereflected waves from flaws.

What is claimed is:
 1. A method for ultrasonic testing of columnarstructures, wherein a surface wave probe is contacted with the surfaceof a rotating columnar structure via a coupling medium existingtherebetween, and surface waves are propagated into the columnarstructure from said surface wave probe while the probe receivesreflected waves from corresponding flaws existing in and just below thesurface of the columnar structure so as to detect said flaws; the methodbeing characterized in that, a center frequency of the surface waves tobe transmitted and received by said surface wave probe is in a frequencyspectrum of said surface waves, and a frequency bandwidth within which apeak value and a peak value−6 dB covers at least 0.50 fc or larger.
 2. Amethod for ultrasonic testing of columnar structures, wherein a surfacewave probe is contacted with the surface of a rotating columnarstructure via a coupling medium existing therebetween, and surface wavesare propagated into the columnar structure from said surface wave probewhile the probe receives reflected waves from corresponding flawsexisting in and just below the surface of the columnar structure so asto detect said flaws; the method being characterized in that the pulseduration of the surface wave pulse which said surface wave probetransmits and receives is at most 2.5 times the period of the surfacewaves that propagate into said columnar structure.
 3. An apparatus forultrasonic testing of columnar structures, wherein a surface wave probeis contacted with the surface of a rotating columnar structure via acoupling medium existing therebetween, and surface waves are propagatedinto the columnar structure from said surface wave probe while the probereceives reflected waves from corresponding flaws existing in and justbelow the surface of the columnar structure so as to detect said flaws;the apparatus is characterized in that said surface wave probe thattransmits and receives said surface waves is provided with a wedge asdisposed on a front surface of a piezoelectric element of the probe andwith a damping block disposed on a back surface of the piezoelectricelement, wherein the pulse length of the surface wave pulse which thesurface wave probe transmits and receives is at most 2.5 times thewavelength of the surface waves that propagate into the columnarstructure.
 4. The apparatus for ultrasonic testing of columnarstructures as claimed in claim 3, wherein said piezoelectric element isany of a lead meta-niobate, a lead titanate, a 1-3 type piezocompositematerial, a 0-3 type piezocomposite material, or a 3-1 typepiezocomposite material.
 5. The apparatus for ultrasonic testing ofcolumnar structures as claimed in claim 3, wherein said wedge has anattenuation coefficient (for longitudinal waves) at 2 MHz of not largerthan 1.8×10² dB/m.
 6. The apparatus for ultrasonic testing of columnarstructures as claimed in claim 5, wherein said wedge is made of apolyimide resin, a polystyrol resin, an acrylic resin, or a fluorineresin.
 7. The apparatus for ultrasonic testing of columnar structures asclaimed in claim 3, wherein said damping block has a volume fraction ofmetallic tungsten powder of at least 40% or larger.
 8. The apparatus forultrasonic testing of columnar structures as claimed in claim 3, whereinsaid wedge has a bottom surface at which it is contacted with thesurface of the columnar structure via a coupling liquid medium existingtherebetween, and has an inclined surface of such that its normal lineintersects the normal line of said bottom surface at an incident angleθi to be defined by the following formula: θi=sin⁻¹(Cw/Cr) where Cwindicates the velocity of the ultrasonic wave in the wedge, and Crindicates the velocity of the surface wave traveling in the columnarstructure, and wherein the front surface of said piezoelectric elementis attached to said inclined surface of the wedge.
 9. The apparatus forultrasonic testing of columnar structures as claimed in any claim 3,wherein said columnar structure is a high-speed tool steel roll.
 10. Theapparatus for ultrasonic testing of columnar structures as claimed inclaim 8, wherein the velocity of the surface waves traveling in thecolumnar structure, Cr in the formula that defines the incident angle θiis a mean value, CRav, of the velocity of the surface wave traveling ineach roll to be tested.
 11. An apparatus for ultrasonic testing ofcolumnar structures, wherein a surface wave probe is contacted with thesurface of a rotating columnar structure via a coupling liquid mediumexisting therebetween, and a surface wave is propagated into thecolumnar structure from said surface wave probe while the probe receivesreflected waves from corresponding flaws existing in and just below thesurface of the columnar structure so as to detect said flaws, theapparatus comprising: columnar structure-rotating means for rotatingsaid columnar structure in the circumferential direction of thestructure, rotating speed-monitoring means for monitoring the rotatingspeed of the columnar structure being rotated by said columnarstructure-rotating means, holder means for holding said surface waveprobe above the columnar structure to ensure a predetermined distancebetween the probe and the surface of the columnar structure, scanningmeans for scanning said holding means in the axial direction of thecolumnar structure, couplant supply means capable of supplying a liquidmedium to be a coupling medium for ultrasonic waves to the gap betweensaid surface wave probe and a surface of the columnar structure andprovided with a flow control valve capable of controlling the flow rateof the liquid medium in accordance with the rotating speed of thecolumnar structure to be rotated by said columnar structure-rotatingmeans, a surface wave probe which is provided with a piezoelectricelement, a wedge disposed on the front surface of a piezoelectricelement and a damping block disposed on the back surface of thepiezoelectric element, so that, where the center frequency of thesurface wave to be transmitted and received by said surface wave probeis fc in the frequency spectrum of the surface waves, a frequencybandwidth within which a spectrum magnitude falls in the range betweenthe peak value and the peak value−6 dB covers at least 0.50 fc orlarger, and that the surface wave probe is capable of detecting theflaws in the columnar structure by use of surface waves, an ultrasonicpulser/receiver capable of supplying said surface wave probe with anelectric pulse for producing surface waves and capable of amplifying thesignals which said surface wave probe has received to a level necessaryfor flaw detection and outputting them, gating means for extracting thesignals for flaw detection from the signals which said ultrasonicpulser/receiver has outputted, and outputting them, and peakdetector/comparator means for detecting the amplitude of the signalswhich the gating means has outputted, and outputting the thus-detectedsignals, or for comparing the amplitude of the signals which the gatingmeans has outputted with a predetermined threshold voltage and, when theamplitude of the signals from the gating means are larger than thepredetermined threshold voltage, outputting signals that indicate thepresence of flaws in the columnar structure being tested.